Using a parameter values in a calculated field

I haven't used field parameters like this before, and I suspect there is a smarter way to do this, but the following seems to be working with your test data at least in a vanilla table visual:

Date_Start = 
VAR _minDE = MIN( 'Table'[DE date] )
VAR _minOE = MIN( 'Table'[OE date] )
VAR _minAll = MIN( _minDE, _minOE )
VAR _selectedDE = CALCULATE( 
    NOT ISEMPTY( Start_date ), 
    FILTER( Start_date, NAMEOF( 'Table'[DE date] ) = Start_date[Start_date Fields] ) 
)
VAR _selectedOE = CALCULATE( 
    NOT ISEMPTY( Start_date ), 
    FILTER( Start_date, NAMEOF( 'Table'[OE date] ) = Start_date[Start_date Fields] ) 
)
RETURN
SWITCH( 
    TRUE(), 
    _selectedDE && _selectedOE, _minAll, 
    _selectedDE, _minDE, 
    _selectedOE, _minOE
)


Date_End = 
VAR _maxPV = MAX( 'Table'[PV date] )
VAR _maxSold = MAX( 'Table'[Sold date] )
VAR _maxAll = MAX( _maxPV, _maxSold )
VAR _selectedPV = CALCULATE( 
    NOT ISEMPTY( End_date ), 
    FILTER( End_date, NAMEOF( 'Table'[PV date] ) = End_date[End_date Fields] ) 
)
VAR _selectedSold = CALCULATE( 
    NOT ISEMPTY( End_date ), 
    FILTER( End_date, NAMEOF( 'Table'[Sold date] ) = End_date[End_date Fields] ) 
)
RETURN
SWITCH( 
    TRUE(), 
    _selectedPV && _selectedSold, _maxAll, 
    _selectedPV, _maxPV, 
    _selectedSold, _maxSold
)


Date_Diff = INT( [Date_End] - [Date_Start] )

Result (note that I made OE date = DE date - 1 as it was harder to verify things were working correctly when they were equal)